1. Field of the Invention
This invention relates to a photo-taking lens having an angle of view of the order of 60.degree. and having a long back focal length, and in particular to a lens for an underwater camera used for underwater photography.
2. Related Background Art
Since a single-lens reflex camera eliminates the problem of parallax and permits its focus to be easily adjusted, attempts to make underwater cameras into single-lens reflex cameras have recently been made. To enable a lens for an underwater camera to be used for a single-lens reflex camera, it is course necessary to make the lens into a construction which can secure a long back focal length. So, to secure a long back focal length in a lens for underwater use as well, the lens is constructed into the heretofore known retrofocus type lens.
When an ordinary retrofocus type lens having an excellent imaging performance in photography in the air and capable of securing a long back focal length is used as a lens for underwater photography, the refractive index of the medium on the object field side varies greatly and therefore aberrations also fluctuate greatly, and it becomes impossible to obtain a sufficient imaging performance. Also, the thickness of the negative lens which is most adjacent to the object side is generally small and therefore, it becomes impossible to neglect influences such as the deformation by water pressure and the like.
On the other hand, there is also known a lens in which a parallel flat plate is disposed on the object field side and is surrounded by a water-proof housing in order that a retrofocus type lens for use in the air may be intactly used for underwater photography. In this case, the focal length and back focal length of that lens during photography in the air do not vary even during underwater photography during which the object field is water, but distortion and chromatic difference of magnification are created by the parallel flat plate inserted forwardly of that lens and fluctuation of aberrations by focusing is also unavoidable.
Lenses for an underwater camera which suppress the fluctuation of aberrations by focusing while having a long back focal length are already known in Japanese Laid-Open Patent Application No. 56-14211 and already Laid-Open Patent Application No. 59-13210. In Japanese Laid-Open Patent Application No. 56-14211, there is proposed a lens in which the center of curvature of the object side surface of a meniscus lens positioned most adjacent to the object side and having its convex surface facing the object side is disposed so as to coincide with the entrance pupil of the entire lens system, whereby off-axis light rays may enter perpendicularly to the object side surface of that lens to thereby make it possible to suppress the variation in the medium on the object field side and the fluctuation of aberrations by focusing. Moreover, in this lens, when the medium on the object field side is water or air, the intergroup spacing is changed to an appropriate spacing in accordance with the medium on the object field side to thereby suppress any fluctuation of the image surface, whereby the lens is made amphibian.
Also, in Japanese Laid-Open Patent Application No. 59-13210, there is proposed a lens exclusively for underwater photography having a long back focal length and yet suppressing the fluctuation of aberrations by focusing and moreover having a wide angle of view of 90.degree.-100.degree..
In the aforementioned Japanese Laid-Open Patent Application No. 56-14211 and 59-13210, it becomes possible to secure a sufficient back focal length as a lens for a single-lens reflex camera, but the former requires eight elements and ten lenses in two lens units and the latter requires nine elements and ten or more lenses, and this leads to a considerably complicated construction which in turn leads to the problem of manufacturing cost.
Also, in the lens disclosed in Japanese Laid-Open Patent Application No. 56-14211, both the forward unit and the rearward unit are complicated in construction and. Moreover when the medium on the object field side changes from water to air or from air to water, the suitable inter-unit spacing must be changed. Therefore, the focusing mechanism becomes considerably complex. Further, both the aforementioned Japanese Laid-Open Patent Application No. 56-14211 and Japanese Laid-Open Patent Application No. 59-13210 have many movable lenses for focusing and the moving weight becomes great. Therefore, in the case of automatic focusing, it becomes difficult to accomplish quick focusing, and this is not preferable.
Also, in underwater photography, absorption of water for red color is great and scattering of light in the water is great, and this may result in reduced contrast. Further, the fact that underwater photography involves many chances for close-up photography gives rise to the necessity of suppressing the fluctuation of aberrations by short distance focusing.